Water base ink composition, method of ink jet recording therewith and record

ABSTRACT

An aqueous ink composition having high storage stability is provided. The aqueous ink composition of the invention contains a coloring agent, a “dispersing resin containing a repeating unit structure (I) having an unneutralized group and a repeating unit structure (II) having a neutralized group and capable of being hydrated and/or dissolved in water”, a water-soluble organic solvent capable of swelling and/or dissolving the foregoing repeating unit structure (I), and water.

TECHNICAL FIELD

The present invention relates to an aqueous ink composition, an inkjetrecording method, and recorded matter.

BACKGROUND ART

The inkjet recording method is a printing method of ejecting smalldroplets of an ink composition and making them adhere to a recordingmedium such as paper, thereby achieving printing. This method ischaracterized in that it is possible to print a high-grade image with ahigh resolution at a high speed by a relatively cheap device.

The ink composition to be used in the inkjet recording is generally onecomprising water as the major component and containing a coloringcomponent and a humectant such as glycerin for the purpose of preventingplugging and the like. As coloring agents that are used in the inkcomposition for inkjet recording, a large number of water-soluble dyesare used because of high saturation of colorants, plenty of the kind ofcolorants that can be utilized, solubility in water, and so on.

On the other hand, however, dyes are often inferior in variouscharacteristics including light fastness and water resistance, andtherefore, printed matters printed from a dye based ink composition areinferior in the light fastness and water resistance. Though the waterresistance is improved by recording paper exclusive for inkjet, whichhas an ink absorbing layer, it is hard to say that the water resistanceis satisfactory with respect to plain paper yet.

Pigments are superior to dyes with respect to the light fastness andwater resistance, and in recent years, for the purpose of improving thelight fastness and water resistance, applications as a coloring agent ofan ink composition for inkjet recording are being investigated. Here,since a pigment is generally insoluble in water, in the case of applyinga pigment to an aqueous ink composition, it is required to mix thepigment together with a dispersant such as water-soluble resins, stablydisperse it in water, and then prepare the dispersion as an inkcomposition.

In order that a pigment may be stably dispersed in the aqueous system,it is necessary to investigate the kind and particle size of thepigment, the kind of a resin to be used, and dispersion means, etc. anda number of dispersion methods and inks for inkjet recording have beenproposed so far. For example, aqueous pigment inks in which carbon blackis dispersed with a surfactant or a high-molecular dispersant are known(see Patent Document 1 and Patent Document 2).

Also, an ink composition comprising water, a styrene-maleic acidcopolymer, ε-caprolactam, and a pigment is proposed (see Patent Document3).

Also, an ink composition containing an aqueous medium, a styrene-maleicacid copolymer, and a copper phthalocyanine pigment is proposed (seePatent Document 4).

Also, an aqueous ink using a resin in which 60% by mole or more of anacid group of a dispersing resin is neutralized with an alkalineneutralizing agent is proposed (see Patent Documents 5 and 6).

Patent Document 1: JP 64-6074 A

Patent Document 2: JP 64-31881 A

Patent Document 3: JP 3-252467 A

Patent Document 4: JP 3-79680 A

Patent Document 5: JP 8-183920 A

Patent Document 6: JP 9-40895 A

For the sake of using a pigment in an ink for inkjet recording asprescribed previously, it is important to stably disperse the pigment inwater and hold it over a long period of time. However, the conventionaltechnologies of the foregoing Patent Document 1 to Patent Document 4were not satisfactory.

Also, a variety of water-soluble organic solvents for optimizing theinkjet recording method are added as an essential component to an ink tobe used in the inkjet recording method in addition to a coloring agent.A humectant to be added for the purpose of preventing drying, apenetration solvent or a surfactant to be added for the purpose ofreducing a surface tension of the ink to control its penetrability intorecording paper, an organic amine for adjusting the pH of the ink, andthe like are added to the ink. Some of these solvents may possiblyinfluence the dispersed coloring agent to inhibit its dispersion,thereby inhibiting stable dispersion of the ink. In particular, solventshaving high affinity with the surface of a pigment having a hydrophobicsurface or a hydrophobic part in the resin involved such a problem thatthese influences are likely revealed.

In the aqueous ink using a resin in which 60% by mole or more of an acidgroup of a dispersing resin is neutralized with an alkaline neutralizingagent as described in the foregoing Patent Document 5 and PatentDocument 6, there was a problem that the resin and a water-solubleorganic solvent to be used are influenced each other, thereby loweringthe storage stability depending upon the kind of the water-solubleorganic solvent.

The invention is aimed to provide an aqueous ink composition having highstorage stability.

DISCLOSURE OF THE INVENTION

The present inventors made extensive and intensive investigations. As aresult, it has been found that the foregoing problems can be solved bythe following constructions.

Specifically, the invention is as described in the following (1) to(17).

(1) An aqueous ink composition containing a coloring agent, a“dispersing resin containing a repeating unit structure (I) having anunneutralized group and a repeating unit structure (II) having aneutralized group and capable of being hydrated and/or dissolved inwater”, a “water-soluble organic solvent capable of swelling and/ordissolving the repeating unit structure (I)”, and water.

(2) The aqueous ink composition as set forth above in (1), wherein thewater-soluble organic solvent is a cyclic amide compound and/or a cyclicurea compound.

(3) The aqueous ink composition as set forth above in (1), wherein thewater-soluble organic solvent is a glycol monoether derivative of apolyhydric alcohol.

(4) The aqueous ink composition as set forth above in (1), wherein thewater-soluble organic solvent is a water-soluble low-molecularmonohydric alcohol.

(5) The aqueous ink composition as set forth above in any one of (1) to(4), wherein the weight of the repeating unit structure (I) is in therange of from 0.05% by weight to 10% by weight based on the weight ofthe water-soluble organic solvent.

(6) The aqueous ink composition as set forth above in any one of (1) to(4), wherein the weight of the repeating unit structure (I) is in therange of from 0.15% by weight to 5% by weight based on the weight of thewater-soluble organic solvent.

(7) The aqueous ink composition as set forth above in any one of (1) to(6), wherein the coloring agent is carbon black.

(8) The aqueous ink composition as set forth above in any one of (1) to(6), wherein the coloring agent is an organic pigment.

(9) The aqueous ink composition as set forth above in any one of (1) to(6), wherein the coloring agent is selected from oil-soluble dyes anddisperse dyes.

(10) The aqueous ink composition as set forth above in any one of (1) to(9), wherein the unneutralized group of the repeating unit structure (I)is a carboxylic acid group and that the neutralized group of therepeating unit structure (II) is a carboxylic acid anion group.

(11) The aqueous ink composition as set forth above in any one of (1) to(10), wherein the repeating unit structure (I) has a molar ratio in therange of from 1% to 67% based on the sum of the repeating unit structure(I) and the repeating unit structure (II).

(12) The aqueous ink composition as set forth above in any one of (1) to(10), wherein the repeating unit structure (I) has a molar ratio in therange of from 1% to 30% based on the sum of the repeating unit structure(I) and the repeating unit structure (II).

(13) The aqueous ink composition as set forth above in any one of (1) to(12), further containing a weakly alkaline agent, wherein thecomposition is alkaline.

(14) The aqueous ink composition as set forth above in (13), wherein theweakly alkaline agent is selected from organic acid salts and organicbuffering agents.

(15) The aqueous ink composition as set forth above in any one of (1) to(14), further containing a water-soluble and/or water-dispersibleaddition resin.

(16) The aqueous ink composition as set forth above in (15), wherein thewater-soluble and/or water-dispersible addition resin has a “repeatingunit structure (I) having an unneutralized group” and a “repeating unitstructure (II) having a neutralized group and capable of being hydratedand/or dissolved in water”.

(17) An inkjet recording method comprising ejecting a droplet of theaqueous ink composition as set forth above in any one of (1) to (16) soas to make the droplet adhere to a recording medium, thereby carryingout recording.

(18) Recorded matter printed with the aqueous ink composition as setforth above in any one of (1) to (16) by an inkjet recording method.

BEST MODE FOR CARRYING OUT THE INVENTION

An aqueous ink composition according to an embodiment of the inventioncontains a coloring agent, a “dispersing resin containing a repeatingunit structure (I) having an unneutralized group and a repeating unitstructure (II) having a neutralized group and capable of being hydratedand/or dissolved in water”, a “water-soluble organic solvent capable ofswelling and/or dissolving the foregoing repeating unit structure (I)”,and water. According to such a construction, it is possible to obtain anaqueous ink composition having high storage stability.

Here, when the dispersing resin does not have a “repeating unitstructure (I) having an unneutralized group”, the requirements of theinvention are not met, and a stable coloring agent dispersion is notobtained. Also, the water-dissolved liberated resin makes ejection ofthe ink unstable. On the other hand, when the dispersing resin does nothave a “repeating unit structure (II) having a neutralized group andcapable of being hydrated and/or dissolved in water”, the resin cannotbe stably dispersed so that the coloring agent dispersion is coagulated.Further, even if the aqueous ink composition contains a “dispersingresin containing a repeating unit structure (I) having an unneutralizedgroup”, when the aqueous ink composition does not contain awater-soluble organic solvent capable of swelling and/or dissolving therepeating unit structure (I), in the case where it is used as an inkjetink, it is impossible to stably eject the ink in the absence of such awater-soluble organic solvent.

The constructional elements of the aqueous ink composition of theinvention will be described below.

Dispersing Resin

The dispersing resin contains a repeating unit structure (I) having anunneutralized group (hereinafter sometimes simply referred to as“repeating unit structure (I)”) and a repeating unit structure (II)having a neutralized group and capable of being hydrated and/ordissolved in water (hereinafter sometimes simply referred to as“repeating unit structure (II)”). The “unneutralized group” as referredto herein means a group capable of being neutralized with a neutralizingagent and includes an acid group and an alkaline group. Specificexamples of the unneutralized group include a carboxylic acid group anda sulfonic acid group. The “neutralized group” as referred to hereinmeans a group resulting from neutralization of an unneutralized groupand is preferably an ion group. The unneutralized group and theneutralized group are preferably an anionic group, and in particular,the case where the unneutralized group is a carboxylic acid group, andthe neutralized group is a carboxylic acid anion group (a group ofcarboxylate) can be suitably enumerated. Examples of carboxylatesinclude lithium carboxylate, sodium carboxylate, potassium carboxylate,and ammonium carboxylate.

For obtaining the dispersing resin, a method of neutralizing a part ofan acid group (unneutralized group) of a dispersing resin precursor of aresin having an anionic group with an alkaline compound such as organicamines and alkali metal salt compounds can be suitably enumerated. Inthis way, it is possible to produce the “dispersing resin containing arepeating unit structure (I) having an unneutralized group and arepeating unit structure (II) having a neutralized group and capable ofbeing hydrated and/or dissolved in water”.

For example, the resin having an anionic group is obtained bypolymerizing a monomer containing an anionic group (hereinafter referredto as “anionic group-containing monomer”) and optionally, other monomercopolymerizable with such a monomer in a solvent. Examples of theanionic group-containing monomer include monomers having a carboxylgroup and monomers having a sulfone group.

As monomers having a carboxyl group, acrylic monomers containing one ortwo carboxyl groups in the repeating unit are preferable.

Specific examples of monomers having a carboxyl group include acrylicacid, methacrylic acid, crotonic acid, ethacrylic acid, propylacrylicacid, isopropylacrylic acid, itaconic acid, fumaric acid, and maleicacid. Of these, acrylic acid, methacrylic acid, and maleic acid arepreferable. It is also preferable that after polymerizing a resin, theresin is sulfonated with a sulfonating agent such as sulfuric acid,fuming sulfuric acid, and sulfamic acid.

As specific examples of monomers having a sulfone group, styrenesulfonicacid, isoprenesulfonic acid, sulfobutyl methacrylate, allylsulfonicacid, and the like are preferable.

Specific examples of other monomers copolymerizable with the anionicgroup-containing monomer include (meth)acrylic esters such as methylacrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butylacrylate, t-butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate,lauryl acrylate, benzyl acrylate, methyl methacrylate, ethylmethacrylate, isopropyl methacrylate, n-propyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexylmethacrylate, n-octyl methacrylate, lauryl methacrylate, stearylmethacrylate, tridecyl methacrylate, and benzyl methacrylate; additionreaction products of a fatty acid and a (meth)acrylic ester monomerhaving an oxirane structure, such as an addition reaction product ofstearic acid and glycidyl methacrylate; addition reaction products of anoxirane compound containing an alkyl group having 3 or more carbon atomsand (meth)acrylic acid; styrene based monomers such as styrene,α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, andp-tert-butylstyrene; itaconic esters such as benzyl itaconate; maleicesters such as dimethyl maleate; fumaric esters such as dimethylfumarate; acrylonitrile, methacrylonitrile, vinyl acetate, isobornylacrylate, isobornyl methacrylate, aminoethyl acrylate, aminopropylacrylate, methylaminoethyl acrylate, methylaminopropyl acrylate,ethylaminoethyl acrylate, ethylaminopropyl acrylate, acrylicaminoethylamide, acrylic aminopropylamide, acrylicmethylaminoethylamide, acrylic methylaminopropylamide, acrylicethylaminoethylamide, acrylic ethylaminopropylamide, methacrylic amide,aminoethyl methacrylate, aminopropyl methacrylate, methylaminoethylmethacrylate, methylaminopropyl methacrylate, ethylaminoethylmethacrylate, ethylaminopropyl methacrylate, methacrylicaminoethylamide, methacrylic aminopropylamide, methacrylicmethylaminoethylamide, methacrylic methylaminopropylamide, methacrylicethylaminoethylamide, methacrylic ethylaminopropylamide, hydroxymethylacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropylmethacrylate, N-methylolacrylamide, and allyl alcohol.

Such a resin preferably has a number average molecular weight in therange of from about 1,000 to 200,000, and especially preferably in therange of from about 3,000 to 150,000. When the number average molecularweight of the resin falls within this range, the resin can fully exhibita function as a coating film in the coloring agent or as a coating inthe ink composition.

It is preferable that such a resin (dispersing resin precursor) and awater-soluble organic solvent described later are chosen in such acombination that the affinity of the dispersing resin precursor with thewater-soluble organic solvent becomes high (the dispersing resinprecursor is swollen or partially dissolved in the water-soluble organicsolvent).

Specific examples of organic amines include salts of a volatile aminecompound such as ammonia, triethylamine, tributylamine,dimethylethanolamine, diisopropanolamine, and morpholine; and salts of asparingly volatile high-boiling organic amine such as diethanolamine,triethanolamine, and tripropanolamine. As specific examples of alkalimetal salt compounds, there are enumerated compounds having lithium,sodium, or potassium as an alkali metal; preferably alkali metalhydroxides such as sodium hydroxide, potassium hydroxide, and lithiumhydroxide; and more preferably potassium hydroxide.

In the invention, the repeating unit structure (I) preferably has amolar ratio in the range of from 1% to 67%, and especially preferably inthe range of from 1% to 30%, based on the sum of the foregoing repeatingunit structure (I) and the foregoing repeating unit structure (II). Inthis way, in particular, it is possible to obtain an aqueous inkcomposition having high storage stability. When the repeating unitstructure (I) is too many, dispersion of the dispersing resin isunstable so that the coloring agent dispersion is likely coagulated.Conversely, when the repeating unit structure (I) is too small, thewhole of the dispersing resin is solubilized in water, and the liberatedresin that does not adhere to the coloring agent increases, whereby aproblem that ejection of the ink becomes unstable is liable to occur.

In the dispersing resin, the amount of the anionic group in the anionicgroup-containing resin is about 30 KOH-mg/g or more, and preferably inthe range of from about 50 to 250 KOH-mg/g in terms of an acid value.When the acid value of the resin falls within such a range, the storagestability of coated coloring agent particles is improved, and the waterresistance of the recorded image is improved.

Water-Soluble Organic Solvent

The ink composition of the invention is constructed such that itcontains a humectant and a penetration solvent or a solvent as describedbelow and additionally, an organic amine for the pH adjustment, and thelike and contains, as an essential component, a water-soluble organicsolvent selected from these materials and capable of swelling and/ordissolving the repeating unit structure (I) of the dispersing resin.

(Humectant)

A humectant is added for the purpose of inhibiting drying of the ink. Itis added for the purpose of inhibiting evaporation of water at thenozzle tip due to drying and preventing coagulation and solidificationof the ink from occurrence.

The humectant is chosen from materials that are soluble in water andhave high hygroscopicity, and useful examples thereof include polyolssuch as glycerin, ethylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol, polyethylene glycol, propylene glycol,dipropylene glycol, polypropylene glycol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 2,3-butanediol,2-methyl-2,4-pentanediol, 2-ethyl-1,3-hexanediol, 1,2,6-hexanetriol, andpentaerythritol; lactams such as 2-pyrrolidone andN-methyl-2-pyrrolidone; and ureas such as 1,3-dimethylimidazolidinone.

Additionally, for the purpose of assisting an ability of the foregoinghumectant organic solvent, it is possible to jointly use or add awater-soluble solid humectant.

Examples thereof include diols such as 1,6-hexanediol, 1,8-octanediol,2,2-dimethyl-1,3-propanediol, and 2,2-diethyl-1,3-propanediol,trimetholethane, trimethylolpropane, lactams such as ε-caprolactam, ureaderivatives such as urea, thiourea, and ethyleneurea, monosaccharides,disaccharides, oligosaccharides, and polysaccharides such as glucose,mannose, fructose, ribose, xylose, arabinose, galactose, aldonic acid,glucitol, (sorbitol), maltose, cellobiose, lactose, sucrose, trehalose,and maltotriose, and derivatives of these sugars such as reducingsugars, oxidizing sugars, amino acids, and thio sugars of the foregoingsugars. In particular, sugar alcohols are preferable, and specificexamples thereof include multitol and sorbitol.

Such a humectant is used singly or in admixture and preferably added inan amount of from 1% by weight to 40% by weight, and more preferablyfrom 1% by weight to 30% by weight in the ink. Such a humectant can beadded in combination with other ink additives in an addition amount suchthat the ink viscosity is not more than 25 cPs at 25° C.

(Penetration solvent)

A penetration solvent is an additive for the purpose of accelerating inkpenetrability into a recording medium and is adequately chosen dependingupon the desired ink drying time.

As one example of the penetration solvent, a water-soluble organicsolvent by which the surface tension of an aqueous solution becomessmall can be chosen from glycol monoether derivatives of a polyhydricalcohol or 1,2-alkyldiols.

As the 1,2-alkyldiols, 1,2-alkyldiols having from 4 to 8 carbon atomssuch as butanediol, pentanediol, hexanediol, heptanediol, and octanediolare preferable. 1,2-Hexanediol, 1,2-heptanediol, and 1,2-octanediol,which are ones having from 6 to 8 carbon atoms, are especiallypreferable because the penetrability into recording paper is strong.Also, it is preferable that the 1,2-alkyldiol is added in an amount inthe range of from 0.25% by weight to 5% by weight.

As the glycol monoether derivatives of a polyhydric alcohol, derivativesof a polyhydric alcohol in which the alkyl moiety thereof has 3 or morecarbon atoms are especially preferable. Specific examples thereofinclude ethylene glycol monobutyl ether, diethylene glycol mono-n-propylether, ethylene glycol monoisopropyl ether, diethylene glycolmonoisopropyl ether, ethylene glycol mono-n-butyl ether, ethylene glycolmono-t-butyl ether, diethylene glycol mono-n-butyl ether, triethyleneglycol mono-n-butyl ether, diethylene glycol mono-t-butyl ether,1-methyl-1-methoxybutanol, propylene glycol mono-t-butyl ether,propylene glycol mono-n-propyl ether, propylene glycol monoisopropylether, propylene glycol mono-n-butyl ether, dipropylene glycolmono-n-butyl ether, dipropylene glycol mono-n-propyl ether, anddipropylene glycol monoisopropyl ether. Also, it is preferable that theglycol monoether derivative of a polyhydric alcohol is added in anamount in the range of from 0.5% by weight to 15% by weight.

Also, as another example of the penetration solvent, a low-boilingwater-soluble organic solvent is preferable, and a water-solublelow-molecular monohydric alcohol is especially preferable. The“water-soluble low-molecular alcohol” as referred to herein means analcohol compound having from 1 to 5 carbon atoms whose solubility inwater (at 20° C.) is from 0.5 wt % to infinity. Examples of such acompound include methanol, ethanol, n-propyl alcohol, isopropyl alcohol,2,2-dimethyl-1-propanol, n-butanol, 2-butanol, tert-butanol, isobutanol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-2-butanol, n-pentanol,2-pentanol, 3-pentanol, and tert-pentanol.

The addition amount of the low-boiling water-soluble organic solvent ispreferably in the range of from 0.5% by weight to 10% by weight based onthe ink composition.

Such a penetration solvent may be used singly, or a plural number ofpenetration solvents can be mixed and used. In particular, by jointlyusing a plural number of compounds having a different structure fromeach other, it is possible to obtain similar images against variouskinds of papers having different absorptivity and color formationproperty, and therefore, such is preferable from the viewpoint ofadaptability to the paper kind.

Incidentally, in the case of an ink with a comparatively long dryingtime, among humectants, a material by which the surface tension of anaqueous solution becomes comparatively small can be used as a substitutefor the penetration solvent. In this case, an ink may be formulatedwithout adding the foregoing penetration solvent.

(Surfactant)

A surfactant is used together with the penetration solvent or usedsingly and is added for the purpose of reducing the surface tension ofthe ink to accelerate the ink penetrability into a recording medium.

Anionic surfactants such as fatty acid salts and alkyl sulfates,nonionic surfactants such as polyoxyethylene alkyl ethers andpolyoxyethylene phenyl ether, acetylene glycol based nonionicsurfactants, cationic surfactants, ampholytic surfactants, silicon basedsurfactants, phosphorus based surfactants, boron based surfactants, andthe like can be used as a suitable material.

Examples of silicon based surfactants include BYK-307, BYK-331, BYK-333,and BYK-348 (trade names, all of which are manufactured by BYK-Chemie).

Further, in the invention, acetylene glycol based nonionic surfactantsare especially preferable because they are low in or free fromfoamability. Specific examples of acetylene glycol compounds that arepreferabled in the invention include Surfynol 61, 82, 104, 440, 465, 485and TG, all of which are manufactured by Air Products and Chemicals,Inc.; and Olfine STG and Olfine E1010, all of which are manufactured byNisshin Chemical Industry Co., Ltd.

Though the addition amount of the acetylene glycol compound may beadequately determined depending upon a desired ink drying time, it ispreferably from 0.01% by weight to 10% by weight based on the inkcomposition.

Incidentally, though the surfactant may be used singly as a penetratingagent, in the case where the surfactant is used together with theforegoing penetration solvent, the whole amount of the penetrating agentcan be decreased, the foaming property of the surfactant can be reduced,and so on, and hence, such is especially preferable.

The water-soluble organic solvent capable of swelling and/or dissolvingthe repeating unit structure (I) of the dispersing resin is preferablyselected from cyclic amide compounds or cyclic urea compounds which areused as the humectant, such as 2-pyrrolidone, N-methyl-2-pyrrolidone,and 1,3-dimethylimidazolidinone, and glycol monoether derivatives of apolyhydric alcohol or water-soluble low-molecular monohydric alcoholswhich are used as the penetration solvent.

Also, it is preferable that the weight of the repeating unit structure(I) of the dispersing resin is in the range of from 0.05% by weight to10% by weight, and preferably in the range of from 0.15% by weight to 5%by weight based on the weight of the foregoing water-soluble organicsolvent.

When the repeating unit structure (I) is too small against thewater-soluble organic solvent, that is, the water-soluble organicsolvent is too many against the repeating unit structure (I), the resinhaving adhered to the coloring agent is dissolved in or peeled by thesolvent, thereby causing such a problem that the dispersion is liable tocause coagulation. Conversely, when the repeating unit structure (I) istoo many against the water-soluble organic solvent, that is, thewater-soluble organic solvent is too small against the repeating unitstructure (I), a problem that fixability of the resin to a recordingmedium becomes worse is liable to occur.

Coloring Agent

As the coloring agent, examples of coloring agents that are insoluble orsparingly soluble in an aqueous medium include organic pigments, carbonblack, oil-soluble dyes, and disperse dyes. In particular, carbon black,organic pigments, oil-soluble dyes, and disperse dyes are preferablebecause they are good in the color formation, and they hardlyprecipitate at the time of dispersion because of their low specificgravity.

In the aqueous ink according to the embodiment of the invention, such acoloring agent is dispersed by the foregoing dispersing resin(preferably, a resin having an anionic group).

With respect to specific examples of carbon black that is preferable inthe invention, examples of carbon black manufactured by MitsubishiChemical Corporation include No. 2300, 900, MCF88, No. 20B, No. 33, No.40, No. 45, No. 52, MA7, MA8, MA100 and No. 2200B. Examples of carbonblack manufactured by Degussa AG include Color Black FW1, FW2, FW2V,FW18, FW200, S150, S160 and S170, Printex 35, U, V and 140U, and SpecialBlack 6, 5, 4A, 4 and 250. Examples of carbon black manufactured byColumbia Carbon Corp. include Conductex SC and Raven 1255, 5750, 5250,5000, 3500, 1255 and 700. Examples of carbon black manufactured by CabotCorporation include Regal 400R, 330R and 660R, Mogul L, Monarch 700,800, 880, 900, 1000, 1100, 1300 and 1400, and Elftex 12. Incidentally,these are merely one example of carbon black that is suitable in theinvention, but it should not be construed that the invention is limitedthereto. Such carbon black may be used singly or in admixture of two ormore thereof. Also, it is preferred to add such a pigment in an amountof from 0.5% by weight to 15% by weight, and preferably from 1% byweight to 10% by weight based on the ink composition.

Examples of organic pigments that are preferable in the inventioninclude quinacridone based pigments, quinacridonequinone based pigments,dioxazine based pigments, phthalocyanine based pigments,anthrapyrimidine based pigments, anthanthrone based pigments,indanthrone based pigments, flavanthrone based pigments, perylene basedpigments, diketopyrrolopyrrole based pigments, perinone based pigments,quinophthalone based pigments, anthraquinone based pigments, thioingigobased pigments, benzimidazolene based pigments, isoindolinone basedpigments, azomethine based pigments, and azo based pigments.

Specific examples of organic pigments that are used in the inkcomposition according to the invention are as follows.

Examples of pigments that are used in a cyan ink composition includeC.I. Pigment Blue 1, 2, 3, 15:3, 15:4, 15:34, 16, 22 and 60; and C.I.Vat Blue 4 and 60. Of these, one member or mixtures of two or moremembers selected from the group consisting of C.I. Pigment Blue 15:3,15:4 and 60 are preferable. Also, such a pigment is contained in anamount of from 0.5% by weight to 15% by weight, and preferably from 1%by weight to 10% by weight based on the cyan ink composition.

Examples of pigments that are used in a magenta ink composition includeC.I. Pigment Red 5, 7, 12, 48 (Ca), 48 (Mn), 57 (Ca), 57:1, 112, 122,123, 168, 184 and 202; and C.I. Pigment Violet 19. Of these, one memberor mixtures of two or more members selected from the group consisting ofC.I. Pigment Red 122, 202 and 209 and C.I. Pigment Violet 19 arepreferable. Also, such a pigment is contained in an amount of from about0.5% by weight to 15% by weight, and preferably from about 1% by weightto 10% by weight based on the magenta ink composition.

Examples of pigments that are used in a yellow ink composition includeC.I. Pigment Yellow 1, 2, 3, 12, 13, 14C, 16, 17, 73, 74, 75, 83, 93,95, 97, 98, 119, 110, 114, 128, 129, 138, 150, 151, 154, 155, 180 and185. Of these, one member or mixtures of two or more members selectedfrom the group consisting of C.I. Pigment Yellow 74, 109, 110, 128 and138 are preferable. Also, such a pigment is contained in an amount offrom about 0.5% by weight to 15% by weight, and preferably from about 1%by weight to 10% by weight based on the yellow ink composition.

Examples of pigments that are used in an orange ink composition includeC.I. Pigment Orange 36 and 43 and a mixture thereof. Also, such apigment is contained in an amount of from about 0.5% by weight to 15% byweight, and preferably from about 1% by weight to 10% by weight based onthe orange ink composition.

Examples of pigments that are used in a green ink composition includeC.I. Pigment Green 7 and 36 and a mixture thereof. Also, such a pigmentis contained in an amount of from about 0.5% by weight to 15% by weight,and preferably from about 1% by weight to 10% by weight based on thegreen ink composition.

In the aqueous ink composition according to the embodiment of theinvention, a ratio of the coloring agent to the dispersing resin ispreferably from 10:1 to 1:10, and more preferably from 4:1 to 1:3. Also,the particle size of the coloring agent at the time of dispersion is notlarger than 300 nm in terms of a mean particle size, and more preferablynot larger than 200 nm in terms of a mean particle size when the maximumparticle size measured by the dynamic light scattering method is lessthan 500 nm.

Weakly Alkaline Agent

In an inkjet recording device, in the case where a metal is used for thepassage of an ink, when the ink is acidic, the metal may possibly becorroded. Accordingly, it is desired that the ink composition of theinvention is adjusted neutral or alkaline.

For the sake of adjusting the ink composition neutral or alkaline, it ispreferable that the ink composition further contains a weakly alkalineagent. Here, for the sake of adjusting the ink neutral or alkaline, if astrongly alkaline compound such as sodium hydroxide is used, in the casewhere the unneutralized group is an acid group (such as a carboxylicacid group and a sulfonic acid group), neutralization of the acid groupmay possibly proceed. In the invention, by using a weakly alkalineagent, it is possible to keep the ink composition neutral or alkalinewithout largely changing the degree of neutralization of the foregoingdispersing resin. Thus, it is possible to provide an ink having higherreliability.

Examples of the foregoing weakly alkaline agent include compoundsselected from organic acid salts and organic buffering agents. As theorganic acid salts, salts of an alkyl carboxylic acid, such as acetatesand propionates, and salts of a hydroxy acid, such as lactates,glycolates, and glycerates, are preferable. Of these, alkali metal saltsof an alkyl carboxylic acid, such as sodium acetate, potassium acetate,sodium propionate, and potassium propionate, are more preferable. As theorganic buffering agents, tris(hydroxymethyl)aminomethane,tris-hydrochlorides, tris-maleic acid, andbis(2-hydroxyethyl)-iminotris(hydroxymethyl)methane are preferable.

The addition amount of the weakly alkaline agent can be adequately setup according to a desired pH of the ink and is preferably in the rangeof from 0.01 to 10% by weight.

Addition Resin

The aqueous ink composition according to the embodiment of the inventioncan further contain a water-soluble and/or water-dispersible additionresin separately from the dispersing resin. It is preferable that afterdispersing the coloring agent with the dispersing resin to prepare adispersion, such an addition resin is added together with thewater-soluble organic solvent and other additives. In general, for thesake of stably dispersing the coloring agent in water, since the surfaceof the coloring agent is hydrophobic, a dispersant such as surfactantsand aqueous resins is essential. The coloring agent according to theinvention is stably dispersed in advance using the dispersing resin, theaddition resin to be added later is not always required to have adispersing ability against the coloring agent. Accordingly, theforegoing dispersing resin may be used as the water-soluble and/orwater-dispersible addition resin, or a water-soluble resin and/or awater-dispersible resin not having a dispersing ability against thecoloring agent may be used.

In the case where the foregoing dispersing resin is used as the additionresin, the addition resin contains a “repeating unit structure (I)having an unneutralized group” and a “repeating unit structure (II)having a neutralized group and capable of being hydrated and/ordissolved in water”. Such an addition resin may contain a monomer thatis the raw material of the dispersing resin.

Also, examples of the water-soluble resin and/or water-dispersibleaddition resin not having a dispersing ability include polyvinylalcohol, polyallyl alcohol, polyhydroxyethyl methacrylate,polyvinylpyrrolidone, polyvinylpyridine quaternary salts,polyacrylamide, carboxymethyl cellulose, hydroxypropyl cellulose,starch, poly-lactic acid, shellac, modified rosin, phenol resins,alkaline salts, and copolymers thereof. Further, resins resulting fromhydrophilization of an oily resin not having a water-soluble group inthe molecule by oxidation, sulfonic acid addition, etc. can be used asthe addition resin.

Water

Water is a medium which becomes the center of the aqueous inkcomposition according to the embodiment of the invention. As preferredwater, for the purpose of reducing ionic impurities as far as possible,pure water such as ion exchange water, ultrafiltration water, reverseosmosis water, and distilled water and ultrapure water can be used.

Also, by using water sterilized upon irradiation with ultraviolet lightor by the addition of hydrogen peroxide, in the case where the inkcomposition is stored over a long period of time, it is possible toprevent the generation of molds or bacteria, and hence, such issuitable.

Other Components

In the aqueous ink composition according to the embodiment of theinvention, additives that are usually used in inks for inkjet recordingcan be further added as the need arises.

Examples of additives that are added as the need arise include a pHadjustor, an antioxidant or an ultraviolet absorber, and an antisepticor an anti-mold agent.

Examples of the pH adjustor include alkali metal hydroxides and amines,such as lithium hydroxide, sodium hydroxide, potassium hydroxide,triethanolamine, diethanolamine, aminomethylpropanol, andtripropanolamine.

Examples of the antioxidant or ultraviolet absorber include allophanatessuch as allophanate and methyl allophanate; biurets such as biuret,dimethyl biuret, and tetramethyl biuret; L-ascorbic acid and saltsthereof; Tinuvin 328, 900, 1130, 384, 292, 123, 144, 622, 770 and 292,Irgacor 252 and 153, Irganox 1010, 1076 and 1035, and MD1024, all ofwhich are manufactured by Ciba-Geigy AG; and oxides of lanthanide.

The antiseptic or anti-mold agent can be selected from, for example,sodium benzoate, pentachlorophenol sodium, 2-pyridinethiol-1-oxidesodium, sodium sorbate, sodium dehydroacetate, and1,2-dibenzothiazolin-3-one.

Dispersion Method of Coloring Agent and Selection of Water-SolubleOrganic Solvent and Dispersing Resin

In the invention, as a suitable method of dispersing the coloring agentwith the dispersing resin, dispersion can be carried out by dissolvingor dispersing an anionic group-containing resin in alkaline watercontaining an alkaline compound such as organic amines and alkali metalsalt compounds, mixing this solution with a coloring agent, anddispersing the mixture using a dispersion machine such as a ball mill, asand mill, an attritor, a roll mill, an agitator mill, a Henschel mixer,a colloid mill, an ultrasonic homogenizer, a jet mill, and an angmill.More preferably, for the sake of firmly bonding the coloring agent tothe resin to stabilize the dispersion, there are employable methodsdisclosed in JP 9-1513142 A, JP 10-140065 A, JP 11-209672 A, JP11-172180 A, JP 10-25440 A, JP 11-43636 A, and JP 2001-247810 A.Outlines of the production methods disclosed in these publications ofapplications will be given below.

JP 2001-247810 A, JP 9-1513142 A, and JP 10-140065 A disclose a “phaseinversion method” and an “acid precipitation method”.

a) “Phase Inversion Method”:

In the invention, the “phase inversion method” as referred to hereinbasically means a self-dispersion (phase inverse emulsification) methodof dispersing a mixed melt of a resin having a self-dispersing abilityor a dissolution ability and a pigment in water. The “mixed melt” asreferred to herein means one including any of the mixed state withoutbeing dissolved, the dissolved and mixed state, or the state includingthe both states.

One specific example includes:

(1) a step of adding a pigment, a neutralizing agent, and a small amountof water to a “dispersing resin precursor (such as the foregoing anionicgroup-containing resin)”/solvent solution, to prepare a solvent baseslurry;

(2) a step of dispersing the slurry while adding it to a large amount ofwater, to prepare a water base slurry; and

(3) a step of removing the solvent used for dissolving the resin fromthe water base slurry, to prepare a pigment-containing polymer particledispersion in which the pigment is included by the water-dispersiblepolymer.

b) “Acid Precipitation Method”:

In the invention, the “acid precipitation method” as referred to hereinmeans a method in which a water-containing cake made of a resin and apigment is prepared, and a part of unneutralized groups contained in theresin in the water-containing cake is neutralized with a neutralizingagent to produce a coloring agent.

Specifically, in the case where the unneutralized group is an anionicacid group, and the neutralizing agent is a basic compound, for example,the acid precipitation method includes (1) a step of dispersing a resinand a pigment in an alkaline aqueous medium and optionally carrying outheat treatment to make the resin gel; (2) a step of making the pHneutral or acidic to render the resin hydrophobic, thereby firmly fixingthe resin to the pigment; (3) a step of optionally carrying outfiltration and water washing to obtain a water-containing cake; (4) astep of neutralizing a part or the whole of anionic groups contained inthe resin in the water-containing cake using a basic compound, followedby re-dispersing in an aqueous medium; and (5) a step of optionallycarrying out heat treatment to make the resin gel.

More specific production methods of the foregoing “phase inversionmethod” and “acid precipitation method” may be the same as thosedisclosed in JP 9-1513142 A and JP 10-140065 A.

Further, JP 11-209672 A and JP 11-172180 A disclose a production methodof the coloring agent. Outlines of this production method basicallycomprise the following production steps.

The production method includes (1) a step of mixing an anionicgroup-containing resin or a solution thereof in an organic solvent witha basic compound to achieve neutralization; (2) a step of mixing thismixed solution with a pigment to form a suspension liquid and thendispersing the pigment by a dispersion machine, etc. to obtain a pigmentdispersion; (3) a step of optionally carrying out distillation to removethe solvent; (4) a step of adding an acidic compound to precipitate theanionic group-containing resin, thereby coating the pigment with theanionic group-containing resin; (5) a step of optionally carrying outfiltration and water washing; and (6) a step of adding a basic compoundto neutralize the anionic group of the anionic group-containing resinand disperse it in an aqueous medium, thereby obtaining an aqueousdispersion.

Also, this production method may be the same as those disclosed in JP11-2096722 A and JP 11-172180 A.

By adding the foregoing water-soluble organic solvent and optionally,the foregoing other components and water to the thus obtained aqueousdispersion of the coloring agent, it is possible to suitably produce anaqueous ink composition. As described previously, it is preferable thatthe water-soluble organic solvent is chosen such that the affinitybetween the foregoing dispersing resin precursor and the water-solubleorganic solvent becomes high (the dispersing resin precursor is swollenor partially dissolved in the water-soluble organic solvent).

As the inkjet recording method of the invention, any method isemployable so far as it is a system in which the foregoing aqueous inkcomposition is ejected as droplets from a fine nozzle so as to make thedroplets adhere to a recording medium. Some of examples thereof will bedescribed below. First of all, an electrostatic suction system isenumerated. This system is a system in which a strong electric field isapplied between a nozzle and an accelerating electrode aligned in frontof the nozzle, an ink is continuously emitted in the droplet state fromthe nozzle, and printing information signals are given to deflectingelectrodes during a period of time when the ink droplets fly between thedeflecting electrodes, to achieve recording, or the ink droplets areemitted without being deflected corresponding to printing informationsignals.

The second method is a system in which a pressure is applied to an inkliquid using a small pump, and a nozzle is mechanically vibrated using acrystal oscillator, etc., thereby forcibly emitting ink droplets. Theemitted ink droplets are electrically charged simultaneously with theemission, and printing information signals are given to deflectingelectrodes during a period of time when the ink droplets fly between thedeflecting electrodes, to achieve recording.

The third method is a system of using a piezoelectric element, in whicha pressure and printing information signals are simultaneously added toan ink liquid using a piezoelectric element, and ink droplets areemitted and recorded.

The fourth method is a system of rapidly volumetrically expanding an inkliquid by the action of heat energy, in which the ink liquid is heatexpanded using microelectrodes according to printing informationsignals, and ink droplets are emitted and recorded.

Among the foregoing various inkjet recording systems, in particular, bycombining the printing method at an ink ejection rate of a relativelylow rate of not more than 10 m/s with the aqueous ink composition of theinvention, it is possible to stably carry out inkjet recording whilepreventing the adhesion of the ink to a ejection nozzle, and hence, suchis preferable.

Also, the recorded matter according to the embodiment of the inventionis obtained by printing with the foregoing aqueous ink composition by aninkjet recording method.

EXAMPLES

The present invention will be illustrated with reference to thefollowing Examples, but the invention should not be construed as beinglimited thereto.

<Preparation of Dispersion>

(1) Dispersion 1:

75 g of MA100 (a trade name, manufactured by Mitsubishi ChemicalCorporation) as carbon black, 25 g of Joncryl 611 (a trade name,manufactured by Johnson Polymer, average molecular weight: 8,100, acidvalue: 53) as a styrene-acrylic acid based water-soluble resin having acarboxylic acid group as an anionic group, 1.0 g of potassium hydroxide,and 250 g of ultrapure water purified by the ion exchange method andreverse osmosis method were mixed, and the mixture was dispersed in aball mill by zirconia beads for 10 hours. The resulting dispersion stocksolution was filtered by a membrane filter (a trade name, manufacturedby Nihon Millipore Ltd.) having a pore size of about 8 μm to removecoarse particles, and the residue was diluted with ultrapure water to apigment concentration of 15 wt % to prepare a dispersion 1 dispersedwith a water-soluble resin.

(2) Dispersion 2:

A dispersion was prepared in the same manner as in the dispersion 1,except that the carbon black was replaced by Color Black S160 (a tradename, manufactured by Degussa AG) and that the addition amount ofpotassium hydroxide was changed to 1.20 g. This liquid is designated asa dispersion 2.

(3) Dispersion 3:

A dispersion was prepared in the same manner as in the dispersion 1,except that the carbon black was replaced by 65 g of C.I. Pigment Blue15:3 that is an organic pigment, that the addition amount of the resinwas changed to 35 g, and that the addition amount of potassium hydroxidewas changed to 1.70 g. This liquid is designated as a dispersion 3.

(4) Dispersion 4:

A dispersion was prepared in the same manner as in the dispersion 1,except that the carbon black was replaced by C.I. Pigment Yellow 74 thatis an organic pigment. This liquid is designated as a dispersion 4.

(5) Dispersion 5:

75 g of MA100 (a trade name, manufactured by Mitsubishi ChemicalCorporation) as carbon black, 25 g of Joncryl 678 (a trade name,manufactured by Johnson Polymer, average molecular weight: 8,500, acidvalue: 215) as a styrene-acrylic acid based water-soluble resin having acarboxylic acid group as an anionic group, 1.80 g of potassiumhydroxide, and 250 g of ultrapure water purified by the ion exchangemethod and reverse osmosis method were mixed, and the mixture wasdispersed in a ball mill by zirconia beads for 10 hours. The resultingdispersion stock solution was filtered by a membrane filter (a tradename, manufactured by Nihon Millipore Ltd.) having a pore size of about8 μm to remove coarse particles, and the residue was diluted withultrapure water to a pigment concentration of 15 wt % to prepare adispersion 5 dispersed with a water-soluble resin.

(6) Dispersion 6:

A dispersion was prepared in the same manner as in the dispersion 1,except that the carbon black was replaced by Color Black S160 (a tradename, manufactured by Degussa AG) and that the addition amount ofpotassium hydroxide was changed to 3.80 g. This liquid is designated asa dispersion 6.

(7) Dispersion 7:

A dispersion was prepared in the same manner as in the dispersion 1,except that the carbon black was replaced by 80 g of C.I. Pigment Red122 that is an organic pigment, that the addition amount of the resinwas changed to 20 g, and that the addition amount of potassium hydroxidewas changed to 3.44 g. This liquid is designated as a dispersion 7.

(8) Dispersion 8:

(Synthesis of Dispersing Resin)

A reactor equipped with a stirrer, a thermometer, a reflux condenser,and a dropping funnel was purged with nitrogen. Thereafter, 25 g ofstyrene, 30 g of n-dodecyl methacrylate, 20 g of methoxy polyethyleneglycol methacrylate, 15.5 g of butyl methacrylate, and 9.3 g ofmethacrylic acid were dissolved in 100 g of methyl ethyl ketone, and thereactor was purged with nitrogen gas. The same solution of monomers inmethyl ethyl ketone was charged in the dropping funnel, 0.2 g of2,2′-azobis(2,4-dimethylvaleronitrile) was further added, and thereactor was purged with nitrogen gas.

Polymerization reaction was carried out in a nitrogen atmosphere byheating at 65° C. while adding the solution in the dropping funnel over3 hours. The resulting copolymer solution was purified by repeatingdrying in vacuo, dissolution in methyl ethyl ketone and filtration, towhich was then added methyl ethyl ketone such that the solids content ofthe resin became 50%. There was thus obtained a resin solution A havingan acid value (KOH) of about 70 and an average molecular weight of50,000.

(Preparation of Dispersion)

150 g of MA100 (a trade name, manufactured by Mitsubishi ChemicalCorporation) as carbon black and 100 g of the foregoing resin solution Awere mixed and stirred to prepare a slurry. 50 g of a 10% KOH aqueoussolution was added to this slurry, and the mixture was dispersed usingan ultrahigh pressure homogenizer.

Subsequently, this dispersion solution was gradually added to 400 g ofpure water with stirring, the whole of the methyl ethyl ketone and apart of the water were removed in vacuo at 60° C., and ultrapure waterwas further added to the residue such that the pigment concentrationbecame 15%. There was thus obtained a dispersion 8.

(9) Dispersion 9:

A dispersion was prepared in the same manner as in the dispersion 8,except that the carbon black was replaced by Color Black S160 (a tradename, manufactured by Degussa AG) and that the addition amount of the10% KOH aqueous solution was changed to 47.5 g. This liquid isdesignated as a dispersion 9.

(10) Dispersion 10:

50 g of C.I. Pigment Blue 15:4 that is an organic pigment and 100 g ofthe foregoing resin solution A were mixed and stirred to prepare aslurry. 47.5 g of a 10% KOH aqueous solution was added to this slurry,and the mixture was dispersed using an ultrahigh pressure homogenizer.

Subsequently, this dispersion solution was gradually added to 250 g ofpure water with stirring, the whole of the methyl ethyl ketone and apart of the water were removed in vacuo at 60° C., and ultrapure waterwas further added to the residue such that the pigment concentrationbecame 15%. There was thus obtained a dispersion 10.

(11) Dispersion 11:

A dispersion was prepared in the same manner as in the dispersion 10,except that the pigment was replaced by 200 g of C.I. Pigment Red 122that is an organic pigment and that the addition amount of the 10% KOHaqueous solution was changed to 39.5 g. This liquid is designated as adispersion 11.

(12) Dispersion 12:

A dispersion was prepared in the same manner as in the dispersion 10,except that the pigment was replaced by 150 g of C.I. Pigment Yellow 74that is an organic pigment and that the addition amount of the 10% KOHaqueous solution was changed to 39.5 g. This liquid is designated as adispersion 12.

(13) Dispersion 13:

(Synthesis of Dispersing Resin)

In a 1-L beaker, 40 wt % of n-butyl methacrylate, 5 wt % of n-butylacrylate, 20 wt % of styrene, 15 wt % of 2-hydroxyethyl methacrylate,and 20 wt % of methacrylic acid were mixed such that the whole amountbecame 500 g, and 4 g of tert-butyl peroxyoctoate as a polymerizationinitiator was further added to obtain a resin synthetic mixed liquid.

Next, 500 g of methyl ethyl ketone was charged in a 1-L flask, and thetemperature was raised to 75° C. while stirring in a nitrogenatmosphere. The foregoing resin synthetic mixed liquid was dropped understirring at 75° C. over 3 hours. Further, the reaction was continuedunder stirring at 75° C. for an additional 8 hours. Thereafter, thereaction synthesis product was spontaneously cooled to 25° C. and thendiluted by the addition of methyl ethyl ketone such that the solidscontent became 50%. There was thus obtained a resin solution B having anacid value (KOH) of 150 and an average molecular weight of 15,000.

(Preparation of Dispersion)

150 g of MA100 (a trade name, manufactured by Mitsubishi ChemicalCorporation) as carbon black, 100 g of the foregoing resin solution B,50 g of a 10% potassium hydroxide aqueous solution, and 700 g ofultrapure water purified by the ion exchange method and reverse osmosismethod were mixed, and the mixture was dispersed together with glassbeads (diameter: 1.7 mm, 1.5 times (by weight) of the mixture) in a sandmill (manufactured by Yasukawa Seisakusho) for 2 hours. Thereafter, theglass beads were eliminated, other components were added, and themixture was stirred at the ambient temperature for 20 minutes, followedby filtration by a 5-μm membrane filter.

The resulting filtrate was subjected to distillation of the whole of themethyl ethyl ketone and a part of the water at 80° C. under atmosphericpressure. Further, a 1N hydrochloric acid solution was dropped whilestirring to coagulate the resin layer. This was subjected to suctionfiltration while washing with water to obtain a water-containing cake ofthe pigment. This water-containing cake was re-dispersed in 375 g of a1% potassium hydroxide aqueous solution while stirring, and ultrapurewater was further added such that the pigment concentration became 15%.There was thus obtained a dispersion 13.

(14) Dispersion 14:

A dispersion was prepared in the same manner as in the dispersion 13,except that the carbon black was replaced by Color Black S160 (a tradename, manufactured by Degussa AG) and that the addition amount of the 1%potassium hydroxide aqueous solution at the time of re-dispersion waschanged to 422 g. This liquid is designated as a dispersion 14.

(15) Dispersion 15:

A dispersion was prepared in the same manner as in the dispersion 13,except that the carbon black was replaced by 250 g of C.I. Pigment Red122 that is an organic pigment and that the addition amount of the 1%potassium hydroxide aqueous solution at the time of re-dispersion waschanged to 422 g. This liquid is designated as a dispersion 15. (16)Dispersion 16:

(Preparation of Dispersing Resin)

50 g of a styrene-methyl methacrylate based resin having an averagemolecular weight of 15,000 and a styrene molar ratio of 35%, 500 g of2-pyrrolidone, and 25 g of sulfamic acid were mixed, and the mixture wasreacted at 80° C. for 3 hours, thereby sulfonating a part of thestyrene. The reaction mixture was purified by repeating dissolution in2-pyrrolidone and precipitation with acidic water at a pH of 5.0 andsubsequently dissolved in methyl ethyl ketone such that the solidscontent became 50%. There was thus obtained a resin solution C having anacid value (KOH) of 70 and an average molecular weight of 15,000.

(Preparation of Dispersion)

150 g of MA100 (a trade name, manufactured by Mitsubishi ChemicalCorporation) as carbon black, 100 g of the foregoing resin solution C,25 g of a 10% potassium hydroxide aqueous solution, and 700 g ofultrapure water purified by the ion exchange method and reverse osmosismethod were mixed, and the mixture was dispersed together with glassbeads (diameter: 1.7 mm, 1.5 times (by weight) of the mixture) in a sandmill (manufactured by Yasukawa Seisakusho) for 2 hours. Thereafter, theglass beads were eliminated, other components were added, and themixture was stirred at the ambient temperature for 20 minutes, followedby filtration by a 5-μm membrane filter.

The resulting filtrate was subjected to distillation of the whole of themethyl ethyl ketone and a part of the water at 80° C. under atmosphericpressure. Further, a 1N hydrochloric acid solution was dropped whilestirring to coagulate the resin layer. This was subjected to suctionfiltration while washing with water to obtain a water-containing cake ofthe pigment. This water-containing cake was re-dispersed in 310 g of a1% potassium hydroxide aqueous solution while stirring, and ultrapurewater was further added such that the pigment concentration became 15%.There was thus obtained a dispersion 16.

(17) Dispersion 17:

A dispersion was prepared in the same manner as in the dispersion 16,except that the carbon black was replaced by 250 g of C.I. Pigment Blue15:3 that is an organic pigment. This liquid is designated as adispersion 17.

<Preparation of Aqueous Ink>

Example 1

26.7 g of the resulting dispersion 1 of carbon black, 20 g of glycerin,5 g of diethylene glycol and 3 g of N-methylpyrrolidone as a humectant,and 3 g of isopropyl alcohol as a penetration solvent were mixed, towhich was then added ultrapure water to make the whole amount to 100 g.The mixture was stirred for 2 hours and filtered by a membrane filter (atrade name, manufactured by Nihon Millipore Ltd.) having a pore size ofabout 1.2 μm to prepare an aqueous ink composition.

Examples 2 to 26

Inks were prepared in the same manner as in Example 1, except that theadditives and amounts were changed to those in formulations shown inTables 1-1 and 1-2.

In Examples 23 and 24, Movinyl 742N (a trade name: Clariant PolymersK.K.) was used as an addition resin 1. In Examples 25 and 26, a resinprepared by using the resin solution A, preparing a resin emulsion of 20wt % of the resin single body in the same manner, except for not addingC.I. Pigment Red 122 to the dispersion 11, and using it in the state ofcontaining a repeating unit structure (I) having an unneutralized groupand a repeating unit structure (II) having a neutralized group andcapable of being hydrated and/or dissolved in water was used as anaddition resin 2.

Comparative Example 1

An ink was prepared by further adding 2.6 g of 1% potassium hydroxide tothe formulation of Example 2. As a result of adding potassium hydroxideat the time of preparing the ink, the amount of potassium hydroxide wasabout 125% based on the amount of the carboxyl group in the dispersingresin.

Comparative Example 2

An ink was prepared by further adding 2.1 g of 1% potassium hydroxide tothe formulation of Example 11. As a result of adding potassium hydroxideat the time of preparing the ink, the amount of potassium hydroxide wasabout 125% based on the amount of the carboxyl group in the dispersingresin.

The foregoing respective formulations are summarized and shown in Tables1-1 and 1-2. TABLE 1-1 Formulation/Example Example 1 2 3 4 5 6 7 8 9 1011 12 Kind of used dispersion Formulation (wt %) 1 2 3 4 5 6 7 8 9 10 1112 Dispersion 26.7 53.3 26.7 40 26.7 46.7 40 26.7 53.3 26.7 40 40Glycerin 20 13 15 15 20 15 15 20 13 15 15 15 Diethylene glycol 5 — — — —— — 5 — — — Triethylene glycol — 7 — 7 5 5 — — 7 2 2 2Trimethylolpropane — — 8 — — — 8 — — 8 8 8 2-Ethyl-1,3-hexanediol — — —— — — — — — — — — 2-Pyrrolidone — 4 — 2 4 — 2 — 2 4 4 4N-Methylpyrrolidone 3 — — — — — — 3 — — — — Isopropyl alcohol 3 — — — 3— — 3 — — — — Triethylene glycol monobutyl ether — — 7 5 — 5 5 — — 1 1 11,2-Hexanediol — — — — — — — — — 3 3 3 2-Butanol — — — — — — — — — — — —Surfynol 465 — 1.5 — 1 — 1 1 — 0.4 0.4 0.4 0.4 Surfynol TG — — — — — — —— 0.5 0.5 0.5 0.5 Surfynol 104 — — — — — — — — — — — Potassiumpropionate — — — — — — — — — — — — Tris(hydroxymethyl)aminomethane — — —— — — — — — — — 1% potassium hydroxide aqueous solution — — — — — — — —— — — — Addition resin 1 — — — — — — — — — — — — Addition resin 2 — — —— — — — — — — — — Ultrapure water Bal. Bal. Bal. Bal. Bal. Bal. Bal.Bal. Bal. Bal. Bal. Bal.Bal: Balance

TABLE 1-2 Formulation/Example Example Com. Ex. 13 14 15 16 17 18 19 2021 22 23 24 25 26 1 2 Kind of used dispersion Formulation (wt %) 13 1415 16 17 1 8 13 10 11 9 11 11 12 2 11 Dispersion 26.7 53.3 40 26.7 4026.7 26.7 26.7 27.6 40 53.3 40 40 40 53.3 40 Glycerin 20 13 15 20 15 2020 20 15 10 10 5 5 5 13 15 Diethylene glycol 5 — — 5 — 5 5 5 — — — — — —— — Triethylene glycol — 7 7 — 7 — — — — — — — — — 7 2Trimethylolpropane — — — — — — — — 10 10 10 10 10 8 — 82-Ethyl-1,3-hexanediol — — — — — — 2 2 4 2 2 2 — — 2-Pyrrolidone — 4 2 —2 — — — 2 2 — 2 2 2 4 4 N-Methylpyrrolidone 3 — — 3 — 3 3 3 — — — — — —— — Isopropyl alcohol 3 — — 3 — 3 3 3 — — — — — — — — Triethylene glycol— — 5 — 5 — — — 2 2 2 1 1 — — 1 monobutyl ether 1,2-Hexanediol — — — — —— — — — — — — 1 — — 3 2-Butanol — — — — — — — — 2 2 — 2 2 3 — — Surfynol465 — 1 1 — 1 — — — — — — — — — 1.5 0.4 Surfynol TG — — — — — — — — — —— — — — — 0.5 Surfynol 104 — — — — — — — — 2 2 1.5 1.5 2.5 2.5 — —Potassium propionate — — — — — 0.5 — — — — — Tris(hydroxymethyl)- — — —— — — 0.5 1 — 5 — 5 — 5 — — aminomethane 1% potassium hydroxide — — — —— — — — — — — — — — 2.6 2.1 aqueous solution Addition resin 1 — — — — —— — — — — — — — — — — Addition resin 2 — — — — — — — — — — 2 13 30 30 —— Ultrapure water Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.Bal. Bal. Bal. Bal. Bal.Bal: Balance<Evaluation method>(Storage stability)

Each of the inks of Examples 1 to 26 and Comparative Examples 1 to 2 wasallowed to stand at 60° C. for 2 weeks and one month, respectively andto stand in the frozen state for one week, and the viscosity immediatelyafter the preparation of an ink was compared with a value afterstanding. The evaluation criteria are as follows:

AA: A fluctuation width is less than ±3%.

A: A fluctuation width is ±3% or more and less than ±6%.

B: A fluctuation width is ±6% or more and less than ±10%.

C: A fluctuation width is ±10% or more.

As a result of the storage stability, a weight ratio “I/S” of the“repeating unit structure (I) having an unneutralized group” to the“repeating unit structure (II) having a neutralized group and capable ofbeing hydrated and/or dissolved in water” and a molar ratio “I/[I+II]”of the “repeating unit structure (I) having an unneutralized group” tothe sum of the foregoing repeating unit structure (I) and the foregoingrepeating unit structure (II) are summarized and shown in Table 2. TABLE2 Weight ratio in Molar ratio in unneutralized part unneutralized part(wt %) (%) Stability Example 1 044 25 AA 2 0.18 10 A 3 0.25 10 AA 4 0.1810 AA 5 4.76 67 A 6 2.13 30 AA 7 1.14 20 AA 8 0.12 5 AA 9 0.23 10 AA 10046 10 AA 11 0.46 25 AA 12 0.58 25 AA 13 2.48 50 AA 14 1.24 25 AA 150.99 25 AA 16 2.48 30 AA 17 0.99 30 AA 18 0.44 25 AA 19 0.12 5 AA 202.48 50 AA 21 0.46 10 AA 22 0.46 25 AA 23 0.23 7 AA 24 0.46 5 AA 25 2.7625 AA 26 2.88 25 AA Comparative Example 1 0 0 C 2 0 0 C

As shown in Table 2, the inks of Examples 1 to 26 each containing a“dispersing resin containing a repeating unit structure (I) having anunneutralized group and a repeating unit structure (II) having aneutralized group and capable of being hydrated and/or dissolved inwater” were stable such that fluctuation in the viscosity before andafter allowing to stand under the foregoing conditions was small as lessthan 6%. On the other hand, in Comparative Example 1 in which in Example2, potassium hydroxide was further added to neutralize all of theunneutralized groups in the resin and Comparative Example 2 in which inExample 11, potassium hydroxide was further added to neutralize all ofthe unneutralized groups in the resin, the stability was inferior suchthat the viscosity started to fluctuate immediately after compoundingwith the materials of ink and that fluctuation in the viscosity beforeand after allowing to stand under the foregoing conditions exceeded 10%.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2002-284270filed Sep. 27, 2002, the contents thereof being herein incorporated byreference.

INDUSTRIAL APPLICABILITY

The aqueous ink composition of the invention has high storage stabilityand is especially suitable for use as an ink for inkjet recording.

1. An aqueous ink composition containing a coloring agent, a “dispersingresin containing a repeating unit structure (I) having an unneutralizedgroup and a repeating unit structure (II) having a neutralized group andcapable of being hydrated and/or dissolved in water”, a “water-solubleorganic solvent capable of swelling and/or dissolving the repeating unitstructure (I)”, and water.
 2. The aqueous ink composition according toclaim 1, wherein the water-soluble organic solvent is a cyclic amidecompound and/or a cyclic urea compound.
 3. The aqueous ink compositionaccording to claim 1, wherein the water-soluble organic solvent is aglycol monoether derivative of a polyhydric alcohol.
 4. The aqueous inkcomposition according to claim 1, wherein the water-soluble organicsolvent is a water-soluble low-molecular monohydric alcohol.
 5. Theaqueous ink composition according to claim 1, wherein the weight of therepeating unit structure (I) is in the range of from 0.05% by weight to10% by weight based on the weight of the water-soluble organic solvent.6. The aqueous ink composition according to claim 1, wherein the weightof the repeating unit structure (I) is in the range of from 0.15% byweight to 5% by weight based on the weight of the water-soluble organicsolvent.
 7. The aqueous ink composition according to claim 1, whereinthe coloring agent is carbon black.
 8. The aqueous ink compositionaccording to claim 1, wherein the coloring agent is an organic pigment.9. The aqueous ink composition according to claim 1, wherein thecoloring agent is selected from oil-soluble dyes and disperse dyes. 10.The aqueous ink composition according to claim 1, wherein theunneutralized group of the repeating unit structure (I) is a carboxylicacid group and that the neutralized group of the repeating unitstructure (II) is a carboxylic acid anion group.
 11. The aqueous inkcomposition according to claim 1, wherein the repeating unit structure(I) has a molar ratio in the range of from 1% to 67% based on the sum ofthe repeating unit structure (I) and the repeating unit structure (II).12. The aqueous ink composition according to claim 1, wherein therepeating unit structure (I) has a molar ratio in the range of from 1%to 30% based on the sum of the repeating unit structure (I) and therepeating unit structure (II).
 13. The aqueous ink composition accordingto claim 1, further containing a weakly alkaline agent, wherein thecomposition is alkaline.
 14. The aqueous ink composition according toclaim 13, wherein the weakly alkaline agent is selected from organicacid salts and organic buffering agents.
 15. The aqueous ink compositionaccording to claim 1, further containing a water-soluble and/orwater-dispersible addition resin.
 16. The aqueous ink compositionaccording to claim 15, wherein the water-soluble and/orwater-dispersible addition resin has a “repeating unit structure (I)having an unneutralized group” and a “repeating unit structure (II)having a neutralized group and capable of being hydrated and/ordissolved in water”.
 17. An inkjet recording method comprising ejectinga droplet of the aqueous ink composition according to claim 1 so as tomake the droplet adhere to a recording medium, thereby carrying outrecording.
 18. Recorded matter printed with the aqueous ink compositionaccording to claim 1 by an inkjet recording method.